Ion exchange membranes have been used as ion-separation membranes in electrodialysis or diffusion dialysis methods for a wide variety of applications such as seawater concentration, removal of salts or nitrate nitrogen from underground brine or brackish water for making drinking water, removal of salts in food production, concentration of active ingredients for medical pharmaceutical product, and others. The ion exchange membranes used for these applications are typically styrene-divinylbenzene type homogeneous ion exchange membranes. Since various technologies have been developed in these membranes for ion selectivity between monovalent and divalent ions, higher selectivity of a specific ion species, reduction in membrane resistance and others, these membranes can provide industrially useful separation techniques.
In the above-mentioned fields such as foods, medical and pharmaceutical products, and agrochemical fields, salts are easily and generally formed as by-products during synthesis of organic compounds. The resulting salts contained in the organic compounds are usually separated using electrodialysis. The electrodialysis desalination can be carried out by alternately arranging a cation exchange membrane and an anion exchange membrane, followed by applying a direct current voltage to electrodes, so that cations travel to the cathode (−) side of the cation exchange membrane and anions travel to the anode (+) side of the anion exchange membrane. As a result, cations and anions are concentrated in concentration chambers each comprising a cation exchange membrane on the cathode side and an anion exchange membrane on the anode side so as to obtain a concentrated electrolyte liquid, and further by removing salts from the concentrated electrolyte liquid, desalination is completed. In the electrodialysis desalination, when electrolytes in liquid to be treated pass through the ion exchange membranes, permeability of ionic species is affected by both ion permeation pathway in the membrane and hydrated ionic radius of the ionic species.
There has been reported a cation exchange membrane comprising a polyvinyl alcohol copolymer. Patent Document 1 describes a cation exchange membrane obtained by casting on a porous support a polymer composition comprising a polystyrene sulfonic acid and a polyvinyl alcohol, followed by thermal-crosslinking of the casted resultant.
Patent Document 2 describes a membrane holding an organic polymeric electrolyte in a three-dimensional network structure formed by cross-linked chains of a polymeric alcohol. Patent Document 2 specifically describes to use a polyvinyl alcohol as the polymeric cross-linkable alcohol, and a polystyrene sulfonic acid as the organic polymeric electrolyte. In order to prevent inhibition of crystal growth in the polyvinyl alcohol caused by heat treatment, sodium bromide contained in the polystyrene sulfonic acid is removed.
Patent Document 3 describes a polymer electrolyte membrane comprising as a main constituent a cross-linked block copolymer comprising a polyvinyl alcohol block and an anionic group-containing polymer block.